[vorbis] Low bitrate encoding

[Gregory Maxwell]

On Tue, Jan 09, 2001 at 03:41:58PM +0100, Dr.Joerg Bergmann wrote:
> > The other detail is what _analogue_ averaging (AKA low pass filtering) is
> > applied. Generally (but not always) frequencies that are above the
> > Nyquist frequency are low pass filtered.
> 
> Thats OK. But sampling itself adds some kind of low pass, an additional
> analogue low pass will only reduce signal/noise ratio. An analogue
> low pass may be (after fourier transformation) described by some
> weighted averaging (weighting function is zero for negative times due to
> signal theory, non-zero for positive values, in general none-zero for all
> future times!)

No. If you sample a signal with signifant components above the nyquest
components these cofeeicents will be aliased and reflected back into the
bandpass of your sampled data. Not cool.

To get good quality downsampling (or A/Ding) you must filter first. This is
non-trivial especially when the rates in noninteger.

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